Wednesday, June 23, 2010

In my high school days I had a part-time job at a professional photo lab / studio called "Phillips' Photo Lab". I was hired as the black and white darkroom guy--I manually processed film, printed custom prints, shot copy negatives, and cleaned the color film and print processors. Later I branched into running the shop on Saturdays and once, just once, did a color print job.

I learned a lot from the pros whose lifeblood was photography: the tips and techniques that make life easier.

One of the things that I would not had believed unless I had personally experienced it was the magic of nose grease. The skin on the nose and to a lesser extent the neck near the back of the ears produces a hydrocarbon of about C30H50 called squalene. The only reason this is important was squalene had the same index of refraction as the gelatin used in photographic processes and hence could be used to fill in scratches on negatives and slides. You would acquire some nose grease, rub it gently into the scratch, and rewash the negative. Nearly as good as new, and it allowed prints and reproductions from otherwise damaged negatives.

This particular example is this blog's first mention of a principle I like to call "matching impedances". In the world there are a lot of interesting issues that in the end are solved or mitigated by taking two different systems and making them the same at the point they meet. In electronics, to maximize power transfer between two parts of a circuit, you match their electrical impedances (which is a function of their resistance at all frequencies).

You have encountered this when setting up a TV or stereo system; the speakers have an impedance rating on them, usually 8ohms for a home stereo and 4 for car audio. Your amplifier speaker outputs need to be designed to deal with a particular impedance; stick a 4ohm speaker on an amp designed for 8 and you might mess things up. On your TVs there used to be two terminals for antennas--one was coaxial and had an impedance of 75ohms and is now the de facto standard, and the other was two screws that had an impedance of 300ohms. To get the most signal you'd want all the impedances to match up: the TV, the cable, and the antenna. If they didn't, part of the precious signal would be reflected back towards the antenna and lost. For converting the impedances to match a little transformer was used, of which there may be a hundred million hiding behind the TVs of the world. Again, here the principle of matching the impedances came into play.

In the optical world reflections occur at surfaces that have differing indexes of refraction. The bigger the difference the higher the reflection ( I think it's a square of (n-n2/n+n2). For solar panels, for instance, the high index of refraction of silicon (4) compared to air (1) makes something like 50% of the incident light reflect off unless you use anti-reflective coatings to improve it. So even for something like a renewable energy resource you have to consider such details.

I use nose grease (n=1.45) to also try to fill in scratches in CDs/DVDs (made of polycarbonate, n=1.58). It's not perfect, but sometimes it makes it playable.

Wednesday, June 09, 2010

The Japanese spacecraft Hayabusa will be returning to Earth with perhaps a sample of the asteroid Itokawa in just four days. This spacecraft has had as many setbacks in the seven years it's been running as you can have without losing the craft completely. When they released the data archive in 2007 I made a color image of Earth made by Hayabusa during a flyby and some more surface close-ups here.